Multiplexers are extensively used in various types of electric and electronic apparatus. Multiplexers selectively output only one signal among a plurality of input signals. One example of a conventional multiplexer is illustrated in FIG. 1, and is briefly described below.
The conventional multiplexer includes a constant current supplying section 10 having first, second and third transistors Q1, Q2 and Q3, and resistors R.sub.1, R.sub.2 and R.sub.3, which supply the constant current. First, second and third comparators 11, 12 and 13 receive the constant current and receive three control signals Sc1, Sc2 and Sc3. The first and second control signals Sc1 and Sc2 are connected to the bases of the fourth and fifth transistors Q4 and Q5 in the first comparator 11, respectively. The third control signal Sc3 is connected to the base of a 6th transistor Q6 in the second comparator 12, and to the base of a 9th transistor Q9 in the third comparator 13. A pair of transistors in each comparator is operated such that only one transistor in each pair is turned on at any given time in accordance with the value of the control signals. That is, if the 4th transistor Q4 is turned on, the 5th transistor Q5 is turned off. Similarly, the transistors of the second and third comparators 12 and 13 operate to ensure that when one transistor of each pair is turned on, the other is turned off.
The first comparator 11 is connected in series with the second and third comparators 12 and 13. Thus, if the 4th transistor Q4 is turned on, then one of the 6th and 7th transistors Q6 and Q7 must also be turned on. Similarly, if the 5th transistor Q5 is turned on, then one of the 8th and 9th transistors Q8 and Q9 must also be turned on. The output of the constant current supplying section 10 is supplied to the bases of the 7th and 8th transistors Q7 and Q8 as a reference voltage, after being voltage-divided by the resistors R4 and R5. Therefore, the "ON/OFF" operations of the 7th and 8th transistors Q7 and Q8 are governed by the voltage level of the third control signal Sc3.
In accordance with the states of the three control signals, various combinations can be formed to allow the multiplexer to perform different operations. Consequently, different output signals can be obtained as will be described in detail below.
In the first example, the first control signal Sc1 has an H (high) state, and the second and third control signals Sc2 and Sc3 have an L (low) state. Thus, the 4th transistor Q4 is turned off, and the 5th transistor Q5 is turned on. Accordingly, current is supplied to the third comparator 13, while no current is supplied to the second comparator 12. In the third comparator 13, the 8th transistor Q8 is turned off, while the 9th transistor Q9 is turned on. Therefore, the constant current from the constant current supplying section 10 is supplied through the 5th and 9th transistors Q5 and Q9 to the bases of the 10th and 14th transistors Q10 and Q14 in a current mirror circuit 14, thereby turning on the 10th and 14th transistors Q10 and Q14.
Meanwhile, first, second, third and fourth input signals Si1, Si2, Si3 and Si4 are respectively supplied to the bases of the 20th, 21st, 22nd and 23rd transistors Q20, Q21, Q22 and Q23 in a signal input/output section 15. When the 14th transistor Q14 is turned on, the 23rd transistor Q23 is also turned on, since the emitter of transistor Q23 becomes low relative to the base. Therefore, the level of the voltage supplied to the bases of 18th and 19th transistors Q18 and Q19 is lowered.
When this voltage level drops sufficiently, the 18th and 19th transistors Q18 and Q19 are turned on, which turns on the 27th transistor Q27. Consequently, the fourth input signal Si4 is obtained in the form of an output signal So. Here, the inverted signal of the fourth input signal Si4 is supplied to the bases of the 18th and 19th transistors Q18 and Q19. A reinverted signal is obtained from the collector of the 19th transistor Q19, with the result that the final output signal So becomes the same as the fourth input signal Si4.
In the second example, the first control signal Sc1 and the third control signal Sc3 have an H state, and the second control signal has an L state. The 5th transistor Q5 of the first comparator 11 is turned on by the first and second control signals, and the 8th transistor Q8 of the third comparator 13 is turned on by the third control signal. Consequently, the current from the constant current supplying section 10 is supplied through the 5th and 8th transistors Q5 and Q8 to the bases of 11th and 15th transistors Q11 and Q15 of the current mirror circuit 14 respectively. Then the 11th and 15th transistors Q11 and Q15 are turned on. Accordingly, the 22nd and 26th transistors Q22 and Q26 of a signal input/output section 15 are turned on, thereby selecting and outputting a third input signal Si3.
In the third example, the first and third control signals Sc1 and Sc3 have an L state, and the second control signal Sc2 has an H state. Accordingly, the first input signal Si1 is output through the output terminal So.
In the fourth example, the first control signal Sc1 has an L state, and the second and third control signals Sc2 and Sc3 have an H state. Therefore, the second input signal Si2 is output through the output terminal So.
The third and fourth cases can be easily understood by referring to the drawings because they control the appropriate transistors in the manner described above for the first and second examples, and therefore, detailed descriptions are omitted.
In a final example, operation of this conventional circuit will be described, in which the first and second control signals have the same state.
First, when the first and second control signals have an H state, the 4th and 5th transistors Q4 and Q5 are both turned off. Therefore, power is not supplied to the second and third comparators 12 and 13. This control signal combination renders the conventional circuit in-operative. Consequently, a signal can not be obtained from the output terminal So.
Further, when the first and second control signals have an L state, the 4th and 5th transistors Q4 and Q5 of the first comparator 11 are both turned on. This state combination causes power to be supplied to the second and third comparators 12 and 13. Under this condition, if the third control signal Sc3 has an L state, the 6th and 9th transistors Q6 and Q9 are turned on. Thus, the 13th and 17th transistors Q13 and Q17 are turned on by the collector current of the 6th transistor Q6, and the 10th and 14th transistors Q10 and Q14 are turned on by the collector current of the 9th transistor Q9. Consequently, the 20th and 24th transistors Q20 and Q24, as well as the 23rd and 27th transistors Q23 and Q27, of the signal input/output section 15 are all turned on. This forms a combination wave, which includes the first input signal Si1 and the fourth input signal Si4, as the output signal.
Further, when the first and second control signals have an L state, and the third control signal has an H state, the 7th and 8th transistors Q7 and Q8 of the second and third comparators 12 and 13 are respectively turned on. These control states causes the 11th and 15th transistors Q11 and Q15, as well as the 12th and 16th transistors Q12 and Q16, of the current mirror circuit 14 to be turned on. Consequently, the 21st and 25th transistors Q21 and Q25, together with the 22nd and 26th transistors Q22 and Q26, of the signal input/output section 15 are all turned on. Thus, a combination wave is again formed, which includes the second input signal Si2 and the third input signal Si3, as the output.
As described above, the multiplexer performs the function of selectively outputting one input signal selected from a plurality of input signals under the control of the combined control signals described in the first to fourth examples. However, in the final example, where the first and second control signals have the same states, the multiplexer malfunctions. In order to prevent such a malfunction, the use of some combinations of the control signals are avoided.
Thus, the conventional multiplexers have a drawback in that malfunctions sometimes occurs, and a further disadvantage is that it operates in an uneconomical manner, since every control signal combination can not be used.